Electromagnetic pulse protective shielding for digital x-ray detectors.

Laser-driven x-ray backlighting can be used to image fast dynamic processes like the propagation of laser-driven shock waves in matter. We demonstrate and evaluate the feasibility of operating the JUNGFRAU detector designed by PSI, a direct detecting x-ray detector, in environments with extreme electromagnetic pulses. The electromagnetic pulse-protective housing is specifically designed for this detector and optimized for pump-probe experiments at the Petawatt High-Energy Laser for Heavy Ion EXperiments (PHELIX) facility at the GSI Helmholtzzentrum für Schwerionenforschung GmbH. The beryllium x-ray entrance window of the protective housing has a high x-ray transmission of 94% at 8 keV. Measurements have shown that the housing simultaneously provides a relative damping of the electromagnetic field on average higher than 1000 in the frequency range of 100 MHz to 5 GHz. The results demonstrate the feasibility of operating digital detectors in experiments where strong electromagnetic pulses are present.

X-ray Dark-Field Imaging for Improved Contrast in Historical Handwritten Literature.

If ancient documents are too fragile to be opened, X-ray imaging can be used to recover the content non-destructively. As an extension to conventional attenuation imaging, dark-field imaging provides access to microscopic structural object information, which can be especially advantageous for materials with weak attenuation contrast, such as certain metal-free inks in paper. With cotton paper and different self-made inks based on authentic recipes, we produced test samples for attenuation and dark-field imaging at a metal-jet X-ray source. The resulting images show letters written in metal-free ink that were recovered via grating-based dark-field imaging. Without the need for synchrotron-like beam quality, these results set the ground for a mobile dark-field imaging setup that could be brought to a library for document scanning, avoiding long transport routes for valuable historic documents.

Noise Reduction for Single-Shot Grating-Based Phase-Contrast Imaging at an X-ray Backlighter.

X-ray backlighters allow the capture of sharp images of fast dynamic processes due to extremely short exposure times. Moiré imaging enables simultaneously measuring the absorption and differential phase-contrast (DPC) of these processes. Acquiring images with one single shot limits the X-ray photon flux, which can result in noisy images. Increasing the photon statistics by repeating the experiment to gain the same image is not possible if the investigated processes are dynamic and chaotic. Furthermore, to reconstruct the DPC and transmission image, an additional measurement captured in absence of the object is required. For these reference measurements, shot-to-shot fluctuations in X-ray spectra and a source position complicate the averaging of several reference images for noise reduction. Here, two approaches of processing multiple reference images in combination with one single object image are evaluated regarding the image quality. We found that with only five reference images, the contrast-to-noise ratio can be improved by approximately 13% in the DPC image. This promises improvements for short-exposure single-shot acquisitions of rapid processes, such as laser-produced plasma shock-waves in high-energy density experiments at backlighter X-ray sources such as the PHELIX high-power laser facility.

Starter substrate specificities of wild-type and mutant polyketide synthases from Rutaceae.

Chalcone synthases (CHSs) and acridone synthases (ACSs) belong to the superfamily of type III polyketide synthases (PKSs) and condense the starter substrate 4-coumaroyl-CoA or N-methylanthraniloyl-CoA with three malonyl-CoAs to produce flavonoids and acridone alkaloids, respectively. ACSs which have been cloned exclusively from Ruta graveolens share about 75-85% polypeptide sequence homology with CHSs from other plant families, while 90% similarity was observed with CHSs from Rutaceae, i.e., R. graveolens, Citrus sinensis and Dictamnus albus. CHSs cloned from many plants do not accept N-methylanthraniloyl-CoA as a starter substrate, whereas ACSs were shown to possess some side activity with 4-coumaroyl-CoA. The transformation of an ACS to a functional CHS with 10% residual ACS activity was accomplished previously by substitution of three amino acids through the corresponding residues from Ruta-CHS1 (Ser132Thr, Ala133Ser and Val265Phe). Therefore, the reverse triple mutation of Ruta-CHS1 (mutant R2) was generated, which affected only insignificantly the CHS activity and did not confer ACS activity. However, competitive inhibition of CHS activity by N-methylanthraniloyl-CoA was observed for the mutant in contrast to wild-type CHSs. Homology modeling of ACS2 with docking of 1,3-dihydroxy-N-methylacridone suggested that the starter substrates for CHS or ACS reaction are placed in different topographies in the active site pocket. Additional site specific substitutions (Asp205Pro/Thr206Asp/His207Ala or Arg60Thr and Val100Ala/Gly218Ala, respectively) diminished the CHS activity to 75-50% of the wild-type CHS1 without promoting ACS activity. The results suggest that conformational changes in the periphery beyond the active site cavity volumes determine the product formation by ACSs vs. CHSs in R. graveolens. It is likely that ACS has evolved from CHS, but the sole enlargement of the active site pocket as in CHS1 mutant R2 is insufficient to explain this process.

Functional expression of cinnamate 4-hydroxylase from Ammi majus L.

Total RNA was isolated from dark-grown cell suspension cultures of Ammi majus L. that had been induced with fungal elicitor or treated with water for control and used as template with cytochrome P450-specific primers for DD-RT-PCR amplifications. A cDNA clone was generated from the elicited transcripts and assigned to cinnamate 4-monooxygenase based on sequence alignments and functional expression in yeast cells. Comparison of the translated polypeptide with database accessions of heterologous cytochrome P450 monooxygenases revealed a high degree of similarity (99.6%) with 98.6% identity to cinnamic acid 4-hydroxylase from parsley, documenting the close evolutionary relationship within the Apiaceae family. Maximal activity of the Ammi hydroxylase in yeast microsomes was determined at 25 degrees C and in the pH range of 6.5-7.0 reaching 2.5 pkat/mg on average. An apparent K(m) of 8.9 microM was determined for cinnamate. Preincubations with psoralen or 8-methoxypsoralen added up to 100 microM in the presence or absence of NADPH hardly affected the turnover rate. A. majus cell cultures accumulate sets of O-prenylated umbelliferones and linear furanocoumarins besides lignin-like compounds upon treatment with elicitor, and cinnamic acid 4-hydroxylase catalyzes the initial reaction leading from the general into the various phenylpropanoid branch pathways. Correspondingly, the hydroxylase transcript abundance was induced in the elicited cells.